Optical fibers with reduced splice loss and methods for making same

Abstract

An optical fiber has a core region and a first cladding region surrounding the core. The first cladding region is doped to increase the fiber's refractive index volume. A second cladding region surrounds the first cladding region. The second cladding region is doped to reduce the fiber's cutoff wavelength, offsetting an increase in the fiber's cutoff wavelength caused by the first cladding region. An outer cladding surrounds the cutoff reduction region. In a further embodiment, the volume increasing region is doped to have a refractive index profile that is sloped to increase from the region's outer circumference towards its inner circumference. In another embodiment, the cutoff reduction region has a step refractive index profile that may have more than one section.

We claim: 
     
       1. An optical fiber, comprising:
 a core region doped to have a positive refractive index relative to the refractive index of an outer cladding region; 
 a volume increasing region surrounding the core, the volume increasing region being doped to have a refractive index that slopes upwardly from a value at its outer circumference equal to that of the outer cladding region to a value at its inner circumference that is substantially lower than that of the core region, such that the volume increasing region increases the fiber's refractive index volume; and 
 a cutoff reduction region surrounding the volume increasing region, the cutoff reduction region being doped to have a negative refractive index, relative to the refractive index of the outer cladding region, such that the cutoff reduction region offsets an increase in the fiber's cutoff wavelength caused by the volume increasing region. 
 
     
     
       2. The fiber of  claim 1 , wherein the cutoff reduction region has a step refractive index profile. 
     
     
       3. The fiber of  claim 2 , wherein the cutoff reduction region has an inner section and an outer section having different refractive indices. 
     
     
       4. The fiber of  claim 3 , wherein the outer section of the cutoff reduction region is doped more heavily than the inner section. 
     
     
       5. The fiber of  claim 1 , wherein the volume increasing cladding region is doped with germanium, and wherein the cutoff reduction region is doped with fluorine. 
     
     
       6. A method for fabricating an optical fiber, comprising:
 fabricating a core region by doping a region of the fiber to have a positive refractive index, relative to the refractive index of an outer cladding region; 
 surrounding the core region with a volume increasing region doped to have a refractive index profile that slopes upwardly from a value at its outer circumference equal to that of the outer cladding region to a value at its inner circumference that is substantially lower than that of the core region, such that the core refractive index volume is increased; and 
 surrounding the volume increasing region with a cutoff reduction region that is doped to have a negative refractive index, relative to the refractive index of the outer cladding region, such that the cutoff region offsets an increase in the fiber cutoff wavelength caused by the volume increasing region. 
 
     
     
       7. The method of  claim 6 , wherein the step of surrounding the volume increasing region with a cutoff reduction region includes doping the cutoff reduction region to have a step refractive index profile. 
     
     
       8. The method of  claim 7 , wherein the step of surrounding the volume increasing region with a cutoff reduction region includes doping the cutoff reduction region to have an inner section and an outer section having different refractive indices. 
     
     
       9. The method of  claim 8 , wherein the outer section is doped more heavily than the inner section. 
     
     
       10. The method of  claim 6 , wherein the volume increasing region is doped with germanium, and wherein the cutoff reduction region is doped with fluorine.